There is need to define those hormonal factors that influence growth of breast cancer. One of these factors, prolactin, has been shown to stimulate growth of certain rat mammary tumors, and is now clearly implicated in the growth of breast cancer in women. It is likely that neoplasms which possess receptors for prolactin, may be treated effectively by removing or interfering with endogenous secretion of prolactin, a situation analogous to that seen with estrogens and specific estrogen receptors. It is the purpose of these studies to define the interaction of prolactin with the cell membrane, the most likely site of prolactin-receptor interaction. The approaches that will be used to characterize hormone receptor interaction are: purification of receptor protein, define those conditions that favor or disfavor hormone-receptor interaction, develop methods for quantitation of hormone receptor in tissue and investigation of the effects of hormones on the prolactin-receptor interaction. Investigations of neoplastic mammary tissue will be performed in rodent tumors, using one model that depends on prolactin for continued tumor growth and a contrasting model that is inhibited in growth by prolactin therapy. Information from these studies will then be applied to human breast cancer as an approach to prediction of those therapeutic regimens that would be most efficacious.